OTTAWA (Reuters)—Canada aims to start immunizing its population against the new H1N1 flu virus in October but has not yet decided who should be a priority for getting the shot, the country's top public health official said on Monday.

"We're hoping that we'll be on track for having the vaccine, start immunizing in late October so that we can complete immunizing before Christmas," Dr. David Butler-Jones, chief public health officer at the Public Health Agency of Canada, told reporters on a conference call.

"There are many things that need to take place between now and then so everything still looks on track, but there's still much work to do," he said.

Drugmakers are gearing up to mass produce vaccines against the new virus, also known as swine flu, and governments have reserved millions of doses as the World Health Organization declared a pandemic.

Yet other countries are not sure yet whether even to give the vaccine to anyone and if so, who should get it. The U.S. Centers for Disease Control and Prevention said on Friday it had not yet decided if it will recommend people get the new shots when flu season arrives in North America this fall.

Dr. Butler-Jones said Canada would try to learn as much as possible about the virus's behavior before deciding how to proceed with an immunization campaign.

"Right now, we're looking at what's happening in the Southern Hemisphere, we're looking at what's happened in Canada ... and the priorities we'll develop ...," he said.

"We won't get the vaccine all at once and we'll be starting to immunize," he said.

The vaccines for seasonal flu will be ready earlier in the fall and some people may need to get two shots, Dr. Butler-Jones said. For the seasonal influenza vaccine, health officials normally prioritize the oldest, the youngest and the sickest. But the majority of those infected with the new virus in Canada are under the age of 20, Dr. Butler-Jones said.

So far, 21 Canadians have died from the new H1N1 flu and 496 have been hospitalized with varying degrees of severity, according to data compiled by the public health agency. Most of those with severe symptoms had some form of underlying health condition.